Aminoacyl-tRNA synthetases will be purified from human placenta and injected into mice in order to generate primed splenic lymphocytes which will be fused to mouse myeloma cells deficient in the enzyme hypoxanthine-guanine phospho-ribosyl transferase. Hybrid cells will be selected in HAT medium, and their secreted products tested for anti-aminoacyl-tRNA synthetase activity by enzyme inhibition studies and enzyme-linked immunosorbent assays. The anti-human tRNA synthetase antibodies obtained will be used in gene-mapping studies and in biochemical studies of aggregated forms of aminoacyl-tRNA synthetases.